1. Field of the Invention
The present invention relates to a raindrop quantity sensing apparatus and a wiper control system having the same.
2. Description of Related Art
In a vehicle, such as an automobile, wiper blades are driven by a wiper motor, so that each of the wiper blades is swung forward from a stop position to a return position and is then swung backward from the return position to the stop position. By repeating the above reciprocal wiping movement of the wiper blades, raindrops on a wiping range of a windshield of the vehicle are wiped.
Japanese Unexamined Patent Publication Number 2000-085538 discloses one such a technique. According to this technique, a quantity of raindrops in a sensing range of a windshield is measured with a raindrop sensor at the time, at which the wiper blade reaches the sensing range, and is thereafter measured with the raindrop sensor once again at the subsequent time, at which the wiper blade reaches the sensing range again. Then, an amount of a change in the quantity of raindrops is computed based on these measurements. Thereafter, a quantity of raindrops, which is measured right before the reaching of the wiper blade to the sensing range, is added to the amount of change. In this way, the quantity of raindrops is measured without a need for providing a raindrop quantity sensing prohibited time period, during which the sensing of the quantity of raindrops with the raindrop sensor is prohibited.
However, when the quantity of raindrops is determined based on the measurement signal of the raindrop sensor, which is obtained while the wiper blade is amid of moving in the sensing range, the state of raindrops in the sensing range may be disturbed by the wiper blade, or the wiped water, which is wiped by the wiper blade and is placed in the sensing range, may be erroneously additionally measured. Thus, in such a case, a quantity of raindrops fell from the sky on the sensing range may not be accurately determined. In view of this, a passing time period, during which the wiper blade moves in the sensing range of the raindrop sensor, is set as a raindrop quantity sensing prohibited time period for prohibiting the sensing of the quantity of raindrops in the sensing range. Also, a time period, during which the wiper blade moves outside of the sensing range, is set as a raindrop quantity sensing execution time period for executing the sensing of the quantity of raindrops in the sensing range. The quantity of raindrops in the sensing range is determined based on the measurement signal of the raindrop sensor, which is outputted in the raindrop quantity sensing execution time period.
FIGS. 11A and 11B show such an example. Here, a range, through which the wiper blade moves during the raindrop quantity sensing execution time period, is set as a raindrop quantity sensing execution range Ap. Also, a range, through which the wiper blade moves during the raindrop quantity sensing prohibited time period, is set as a raindrop quantity sensing prohibited range Ab. The raindrop quantity sensing execution range Ap is separated from the raindrop quantity sensing prohibited range Ab at a boundary Z. In FIGS. 11A and 11B, the raindrop quantity sensing prohibited range Ab includes a range other than the sensing range Ad of the raindrop sensor for the following reason. Specifically, the wiping level of wiper blade 10 may be sequentially changed to the intermittent drive operation at the long intermittent time interval, the intermittent drive operation at the short intermittent time interval, the continuous drive operation at the low wiping speed and the continuous drive operation at the high wiping speed depending on a need. The intermittent time interval and the wiping speed may vary from a vehicle to vehicle regardless of a vehicle model (regardless of the same model or different models). Furthermore, the raindrop sensor is placed in the predetermined location in the windshield. Regardless of the vehicle model, the location of the raindrop sensor may vary. Thus, the wiping speed of the wiper blade 10 and the distance from the stop position to the sensing range Ad of the raindrop sensor may vary from the vehicle to vehicle, so that it is difficult to accurately set the time period, during which the wiper blade 10 moves in the sensing range Ad, as the raindrop quantity sensing prohibited time period. In order to reliably include the time period, during which the wiper blade 10 moves in the sensing range Ad, into the raindrop quantity sensing prohibited time period, and also to tolerate the above variations, the raindrop quantity sensing prohibited time period includes the time period, during which the wiper blade 10 moves outside the sensing range Ad. Thus, the raindrop quantity sensing prohibited range Ab includes the range other than the sensing range Ad of the raindrop sensor.
Now, a sensing principle of the raindrop sensor will be described. The raindrop sensor includes a light emitting device and a light receiving device. The light emitting device may be a light emitting diode, which outputs an infrared light toward the sensing range Ad in the wiping range Aw of the windshield. The light receiving device receives a reflected light, which is outputted from the light emitting device and is reflected from the windshield, and outputs a measurement value, which corresponds to a quantity of the received light. When no raindrop is present in the sensing range Ad, the infrared light, which is outputted from the light emitting device, is substantially entirely reflected by the windshield (total reflection) and is received by the light receiving device. However, when raindrops are present in the sensing range Ad, a portion of the infrared light, which is outputted from the light emitting device, penetrates through the windshield through the raindrops present in the sensing range Ad. Thus, the amount of light, which is received by the light receiving device, is reduced. Specifically, when the quantity of raindrops in the sensing range Ad is reduced, the amount of light, which penetrates through the windshield, is reduced. Thus, in such a case, the measurement signal of the raindrop sensor is increased. In contrast, when the quantity of raindrops in the sensing range Ad is increased, the amount of light, which penetrates through the windshield, is increased. Thus, in such a case, the measurement signal of the raindrop sensor is reduced.
The sensing operation of the raindrop sensor will be now described further with reference to FIGS. 12A to 12D. With reference to FIG. 12A, the wiper motor begins to rotate at the time t10 to swing the wiper blade 10 and is stopped at the time t14. Thereafter, the wiper motor begins to rotate at the time t15 to swing the wiper blade 10 and is stopped at the time t19. In each operating state, i.e., ON-state of the wiper motor (a period between the time t10 and the time t14, a period between the time t15 and the time t19), the wiping range Aw is wiped by the wiper blade 10. In each stop state, i.e., OFF-state of the wiper motor (before the time t10, a period between the time t14 and the time t15, and after the time t19), the wiper blade 10 is stopped in the stop position (FIG. 11).
As shown in FIG. 12B, the wiper blade 10 is moved from the raindrop quantity sensing execution range Ap into the raindrop quantity sensing prohibited range Ab at the time t11 (end of the raindrop quantity sensing execution time period) that is reached after elapse of a predetermined time period ΔT1 from the time t10, at which the wiper motor is shifted from the stop state to the operating state. Then, for example, the wiper blade 10 is swung forward to pass the sensing range Ad at the time t12 and is returned at the return position. Thereafter, the wiper blade 10 is swung backward to pass the sensing range Ad at the time tb once again. Furthermore, the wiper blade 10 is moved from the raindrop quantity sensing prohibited range Ab into the raindrop quantity sensing execution range Ap at the time ts (beginning of the raindrop quantity sensing execution time period), which is reached after elapse of a predetermined time period ΔT2 from the time t10 described above. Similarly, the wiper blade 10 is moved from the raindrop quantity sensing execution range Ap into the raindrop quantity sensing prohibited range Ab at the time te (end of the raindrop quantity sensing execution time period) that is reached after elapse of the predetermined time period ΔT1 from the time t15, at which the wiper motor is shifted from the stop state to the operating state once again. Next, for example, the wiper blade 10 passes the sensing range Ad at the time t16 and is returned at the return position. Then, the wiper blade 10 passes the sensing range Ad at the time t17 once again. Then, the wiper blade 10 is moved from the raindrop quantity sensing prohibited range Ab into the raindrop quantity sensing execution range Ap at the time t18 (beginning of the raindrop quantity sensing execution time period), which is reached after elapse of the predetermined time period ΔT2 from the time t15 described above. The predetermined time period ΔT1 and the predetermined time period ΔT2 are set each time depending on the wiping speed of the wiper blade 10.
FIG. 12C shows the shift of the measurement signal of the raindrop sensor in a state where moderate rain shower (moderate rainfall) exists. As shown in FIG. 12C, the measurement signal, which is outputted from the raindrop sensor, changes with time. Specifically, the quantity of raindrops in the sensing range Ad is increased with time, so that the measurement signal, which is outputted from the raindrop sensor, is gradually reduced. At the time t12, at which the wiper blade 10 passes the sensing range Ad, the scraped water, which is scraped by the wiper blade 10, as well as the adhered water, which is adhered to the wiper blade 10, are brought into the sensing range Ad to cause an increase in the amount of light, which is outputted from the light emitting device and penetrates through the windshield. Thus, the measurement signal of the raindrop sensor largely drops once. Right after the time t12, the sensing range Ad is wiped by the wiper blade 10, so that the amount of light, which is outputted from the light emitting device and penetrates through the windshield, becomes substantially zero. Thus, the measurement signal of the raindrop sensor is largely increased. At the time tb, at which the wiper blade 10 passes the sensing range Ad upon returning from the return position, the scraped water, which is scraped by the wiper blade 10, is relatively small due to the small distance from the return position but causes the decrease of the measurement signal of the raindrop sensor once again. Right after the time tb, the measurement signal of the raindrop sensor is increased to the level, which is the same as that of the time t12. Thereafter, the measurement signal of the raindrop sensor is gradually decreased. Thus, as long as the same level of rain shower exists, the measurement signal of the raindrop sensor basically shows the above shift.
However, when the temperature of the raindrop sensor changes, the above shift may not be maintained in some situations even in the case where the same level of rain shower exits. Specifically, the raindrop sensor has a temperature characteristic of changing the measurement signal thereof due to luminance characteristic of the light emitting device of the raindrop sensor. That is, in the case where the same quantity of raindrops is present in the sensing range Ad, when the temperature decreases, the measurement signal of the raindrop sensor increases, and vice versa. Therefore, even when one-to-one relationship between the quantity of raindrops in the sensing range Ad and the measurement signal of the raindrop sensor is preset, this relationship changes upon changing of the temperature of the raindrop sensor. Thus, it is not possible to accurately determine the quantity of raindrops in the sensing range Ad based directly on the measurement signal received from the raindrop sensor.
Therefore, according to the previously proposed technique, the amount of change ΔV is computed by subtracting the measurement signal Vb of the raindrop sensor at the end (time te) of the raindrop quantity sensing execution time period from the measurement signal Va of the raindrop sensor at the beginning (time ts) of the raindrop quantity sensing execution time period. The quantity of raindrops in the sensing range Ad is determined based on this amount of change ΔV. In this way, even when the temperature of the raindrop sensor is changed, the quantity of raindrops in the sensing range Ad can be determined. The principle of this determination will be described below.
For example, it is assumed that the state of rain is changed from the moderate rain shower to heavy rain shower (large rainfall). In such a case, the quantity of raindrops fell from the sky on the sensing range Ad per unit time is increased. Thus, under the heavy rain shower, the measurement signal Va of the raindrop sensor at the time ts is reduced in comparison to the measurement signal Va of the raindrop sensor at the time ts under the moderate rain shower. Similarly, the measurement signal Vb of the raindrop sensor at the time te under the heavy rain shower is reduced in comparison to the measurement signal Vb of the raindrop sensor at the time te under the moderate rain shower. That is, the measurement signal Va and the measurement signal Vb are both reduced at the time ts and at the time te. In general, the time interval between the time ts and the time te is longer than a time interval between the time tb and the time ts. Thus, when the rainfall is increased, the quantity of raindrops fell from the sky on the sensing range Ad during the time interval between the time ts and the time te is increased in comparison to the quantity of raindrops fell from the sky on the sensing range Ad during the time interval between the time tb and the time ts. When the quantity of raindrops fell from the sky on the sensing range Ad is increased, the measurement signal of the raindrop sensor is reduced. Thus, when the state of rain changes from the moderate rain shower to the heavy rain shower, the amount of decrease in the measurement signal Vb of the raindrop sensor becomes greater than the amount of decrease in the measurement signal Va. In other words, the measurement signal Vb of the raindrop sensor shows the greater decrease in comparison to the measurement signal Va of the raindrop sensor. Thus, when the state of rain changes from the moderate rain shower to the heavy rain shower, the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period is increased. The amount of change ΔV in the measurement signal is also increased when the state of rain changes from the no-rain state to light rain shower (small rainfall) or changes from the small rain shower to the moderate rain shower.
Now, for example, it is assumed that the state of rain is changed from the moderate rain shower to the light rain shower (small rainfall). In such a case, the quantity of raindrops fell from the sky on the sensing range Ad per unit time is decreased. Thus, under the light rain shower, the measurement signal Va of the raindrop sensor at the time ts is increased in comparison to the measurement signal Va of the raindrop sensor at the time ts under the moderate rain shower. Similarly, the measurement signal Vb of the raindrop sensor at the time te under the light rain shower is increased in comparison to the measurement signal Vb of the raindrop sensor at the time te under the moderate rain shower. That is, the measurement signal Va and the measurement signal Vb are both increased at the time ts and at the time te. In general, the time interval between the time ts and the time te is longer than the time interval between the time tb and the time ts. Thus, when the rainfall is decreased, the quantity of rain not fell from the sky on the sensing range Ad between the time interval between the time ts and the time te is increased in comparison to the quantity of rain not fell from the sky on the sensing range Ad in the time interval between the time tb and the time ts. In other words, the measurement signal Vb of the raindrop sensor shows the greater increase in comparison to the measurement signal Va of the raindrop sensor. Thus, when the state of rain changes from the moderate rain shower to the light rain shower, the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period is decreased. The amount of change ΔV in the measurement signal is also decreased when the state of rain changes from the heavy rain shower to the moderate rain shower or changes from the small rain shower to the no-rain state.
Furthermore, although the time interval between the time ts and the time te is relatively long, it is not long enough to cause a substantial change in the temperature of the raindrop sensor. Thus, the temperature Ta of the raindrop sensor at the time ts can be considered as generally the same temperature as the temperature Tb of the raindrop sensor at the time te. Therefore, according to the previously proposed technique, regardless of the temperature of the raindrop sensor, whenever the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period is increased, it is determined that the rainfall is increased. Thus, the wiping level of the wiper blade 10 is increased. In contrast, whenever the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period is reduced, it is determined that the rainfall is reduced. Thus, the wiping level of the wiper blade 10 is reduced.
When the rainfall is shifted in the order of the small rainfall, the moderate rainfall and the large rainfall and is thereafter increased to an extremely large rainfall, the following disadvantage is encountered. FIG. 12D shows the shift of the measurement signal of the raindrop sensor in the case of the extremely large rainfall.
Specifically, at the time of the extremely large rainfall, the amount of raindrops fell from the sky on the sensing range Ad per unit time is extremely large. Thus, the measurement signal Va of the raindrop sensor at the time ts under extremely heavy rainfall becomes smaller than the measurement signal Va of the raindrop sensor at the time ts under the large rainfall. However, the measurement signal Vb of the raindrop sensor at the time te under the extremely heavy rainfall is not significantly reduced in comparison to the measurement signal Vb of the raindrop sensor at the time te under the large rainfall. That is, at the time of the extremely large rainfall, the entire surface area of the sensing range Ad is quickly covered with the large quantity of raindrops between the time tb and the time ts. Thus, even when the raindrops are kept applied to the sensing range Ad, the amount of light, which is outputted from the light emitting device and penetrates through the windshield, cannot increase largely from the time ts to the time te. Therefore, in the case where the state of rain is changed from the heavy rain shower (the heavy rainfall) to the extremely heavy rain shower (the extremely heavy rainfall), although the measurement signal Va and the measurement signal Vb are reduced at the time ts and the time te, respectively, the measurement signal Va shows the greater degrease in comparison to the measurement signal Vb. Thus, the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period becomes small. Therefore, according to the previously proposed technique, which uses the amount of change ΔV in the measurement signal of the raindrop sensor in the raindrop quantity sensing execution time period, it is not possible to distinguish the shifting of the rainfall in the order of the large rainfall, the moderate rainfall, the small rainfall and no rainfall from the shifting of the rainfall from the large rainfall to the extremely large rainfall. In such a case, the wiping level of the wiper blade 10 could possibly be reduced at the time of the extremely heavy rainfall where the highest wiping level of the wiper blade 10 is required.